Closes #4201.

… r=Veykril

Handle attributes correctly in "Flip comma"

Attributes often contain path followed by a token tree (e.g. `align(2)`), and the previous code handled them as two separate items, which led to results such as `#[repr(alignC, (2))]`.

An alternative is to just make the assist unavailable in attributes, like we do in macros. But contrary to macros, attributes often have a fixed form, so this seems useful.

Fixes rust-lang#18013.